Smoking and cancer: What’s in a cigarette?
A cigarette may look harmless enough – tobacco leaves covered in classic white paper. But when it burns, it releases a dangerous cocktail of about 4,000 chemicals including:
- more than 70 cancer-causing chemicals
- hundreds of other poisons.
- nicotine, a highly addictive drug, and many additives designed to make cigarettes taste nicer and keep smokers hooked.
This page has more information on the various poisons in cigarette smoke. You can also read about where these come from and how concentrated they actually are.
Cancer-causing chemicals in tobacco smoke
- Tar – a mixture of dangerous chemicals
- Arsenic – used in wood preservatives
- Benzene – an industrial solvent, refined from crude oil
- Cadmium – used in batteries
- Formaldehyde – used in mortuaries and paint manufacturing
- Polonium-210 – a highly radioactive element
- Chromium – used to manufacture dye, paints and alloys
- 1,3-Butadiene – used in rubber manufacturing
- Polycyclic aromatic hydrocarbons – a group of dangerous DNA-damaging chemicals
- Nitrosamines – another group of DNA-damaging chemicals
- Acrolein – formerly used as a chemical weapon
- Other chemicals
Other poisons in cigarette smoke
- Hydrogen cyanide – used as an industrial pesticide
- Carbon monoxide – found in car exhausts and used in chemicals manufacturing
- Nitrogen oxides – a major component of smog
- Ammonia – used to make fertilisers and explosives
- More poisons
Tar is a term that describes a collection of solid particles that smokers inhale when they light a cigarette. It is a mixture of lots of chemicals, many of which can cause cancer. When it settles, tar forms a sticky, brown residue that can stain smokers’ teeth, fingers and lungs.
Because tar is listed on packs, it is easy to believe that it is the only harmful part of cigarettes. But some of the most dangerous chemicals in tobacco smoke are present as gases, and do not count as part of tar. This means that cigarettes with less tar still contain all the other toxic chemicals.
Arsenic is one of the most dangerous chemicals in cigarettes. It can cause cancer as well as damaging the heart and its blood vessels.
Small amounts of arsenic can accumulate in smokers’ bodies and build up to higher concentrations over months and years. As well as any direct effects, it can worsen the effect of other chemicals by interfering with our ability to repair our DNA.
Fish and seafood can be major sources of arsenic, but in a form that is less toxic and more readily removed from the body. In contrast, tobacco smoke contains arsenic in a more dangerous form.
Benzene is a solvent used to manufacture other chemicals, including petrol. It is well-established that benzene can cause cancer, particularly leukaemia. It could account for between a tenth and a half of the deaths from leukaemia caused by smoking.
Tobacco smoke contains large amounts of benzene and accounts for a big proportion of our exposure to this poison. The average smoker inhales about ten times more benzene than the average non-smoker.
And some studies have estimated that the amount of benzene that a person inhales through second-hand smoke over their lifetime could increase their risk of cancer.
Cadmium is a metal used mostly to make batteries. The majority of cadmium in our bodies comes from exposure to tobacco smoke. Smokers can have twice as much cadmium in their blood as non-smokers.
Studies have found that the amounts of cadmium present in tobacco smoke are capable of affecting our health. It is a known cause of cancer, and can also damage the kidneys and the linings of the arteries.
Our bodies have proteins that mop up harmful chemicals like cadmium, but the amounts in smoke can overload these proteins. Cadmium can also prevent our cells from repairing damaged DNA. Because of this, it can make the effects of other chemicals even worse.
Formaldehyde is a smelly chemical used to kill bacteria, preserve dead bodies and manufacture other chemicals. It is one of the substances in tobacco smoke most likely to cause diseases in our lungs and airways.
Formaldehyde is also a known cause of cancer. It is believed that even the small amounts in second-hand smoke could increase our lifetime risk of cancer.
Tobacco smoke is one of our major sources of formaldehyde exposure. Places where people smoke can have three times the normal levels of this poison.
Polonium is a rare, radioactive element and polonium-210 is its most common form. Polonium strongly emits a very damaging type of radiation called alpha-radiation that can usually be blocked by thin layers of skin.
But tobacco smoke contains traces of polonium, which become deposited inside their airways and deliver radiation directly to surrounding cells.
The lungs of smokers can be exposed to four times more polonium than those of non-smokers and specific parts may get a hundred times more radiation. One study estimated that someone smoking one and half packs a day receives the equivalent amount of radiation as someone having 300 chest X-rays a year.
Chromium is a metal used to make metallic alloys, dyes and paints and comes in different types. Chromium III or ‘trivalent chromium’ is most commonly used. It is available as dietary supplements and is harmless.
On the other hand, chromium VI or ‘hexavalent chromium’ is very toxic, is found in tobacco smoke, and is known to cause lung cancer. It allows other cancer-causing chemicals (such as polycyclic aromatic hydrocarbons) to stick more strongly to DNA and damage it.
1,3-butadiene or BDE is an industrial chemical used in rubber manufacture. Some scientists believe that of all the chemicals in tobacco smoke, BDE may present the greatest overall cancer risk. It may not be as good at causing cancer as some of the other chemicals listed here, but it is found in large amounts in tobacco smoke.
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons or PAHs are a group of powerful cancer-causing chemicals that candamage DNA and set cells down the road to becoming tumours.
One of these chemicals – benzo(a)pyrene or BAP – is one of the most widely studied of all tobacco poisons. BAP directly damages p53, a gene that normally protects our bodies against cancer.
Nitrosamines are a group of chemicals that can directly damage DNA, like polycyclic aromatic hydrocarbons (PAHs).
They are found in small amounts in food. But tobacco products, including those that are chewed rather than smoked, are by far our largest source of exposure to these chemicals. Even though they are found in relatively small amounts in cigarettes, they are very strong cancer-causing chemicals.
Acrolein is a gas with an intensely irritating smell and is one of the most abundant chemicals in cigarette smoke. It belongs to the same group of chemicals as formaldehyde and acetaldehyde, both of which can cause cancer.
Until now, it wasn’t clear if acrolein causes cancer as well, but recent experiments suggest that it can. We now know that acrolein can cause DNA damage that is similar to the damage seen in lung cancer patients. Since smoke contains up to 1,000 times more acrolein than other DNA-damaging chemicals, it could be a major cause of lung cancer.
Acrolein also stops our cells from repairing DNA damage, like arsenic and cadmium. And like hydrogen cyanide, it kills the hairs that normally clean our lungs of other toxins.
Some of the other cancer-causing ingredients of tobacco smoke include:
- metals, such as nickel, lead, cobalt and beryllium. While you may be exposed to some of these metals through domestic items or food, inhaling them in tobacco smoke is worse, because they are easily absorbed by the lungs.
- acetaldehyde, which is also formed in your tissues when you drink alcohol – it is responsible for many nasty hangover symptoms
- hydrazine, a very toxic chemical used mainly in rocket fuel
Hydrogen cyanide is a poisonous gas. Of all the chemicals in tobacco smoke, it does the most damage to the heart and blood vessels.
Hydrogen cyanide does not cause cancer, but it increases the risk of other chemicals causing cancer by damaging cilia. These are tiny hairs lining the airways that help to clear toxins away. By killing cilia, hydrogen cyanide causes other dangerous chemicals to be stuck in the lungs and airways.
Carbon monoxide is a colourless gas with no smell. It is formed when we burn carbon-based fuels, such as gas in cookers or petrol in car engines. It can make up as much as 3-5% of tobacco smoke.
Carbon monoxide sticks to our red blood cells in place of oxygen. This lowers our blood’s ability to transport oxygen and deprives our tissues and organs of this vital gas.
Like hydrogen cyanide, it kills cilia and reduces our lungs’ ability to clear away toxins. This means that while carbon monoxide does not cause cancer directly, it makes it easier for other chemicals to do so.
Nitrogen oxide is a gas found in car exhaust and tobacco smoke.
Our bodies use it in very small amounts to carry signals between cells. But in large amounts, it is a major air pollutant. It directly damages lung tissue and causes inflammation in the lungs.
Normally, our bodies produce small amounts of nitrogen oxide, which causes our airways to expand. The large amount of nitrogen oxide in tobacco smoke changes things in two ways:
- When smokers are smoking, it expands their airways even further, making it easier for their lungs to absorb nicotine and other chemicals.
- When they are not smoking, it shuts off their internal nitrogen oxide production line, causing their airways to constrict. This is one reason why regular smokers often have difficulty breathing.
Ammonia is a gas with a strong, irritating smell, and is used in some toilet cleaners. Some studies have found that ammonia enhances the addictive power of nicotine. It changes nicotine into a gas that is more readily absorbed into the lungs, airways and bloodstream.
Like carbon monoxide and hydrogen cyanide, ammonia also kills cilia.
Tobacco smoke also contains many other poisons that produce harmful effects. These can be carried throughout the body via our blood vessels.
As well as hydrogen cyanide and ammonia, gases like sulphur dioxide also kill cilia (protective hairs) in our lungs. This stops them from being able to clear away other harmful chemicals.
Chemicals like hydrogen sulphide and pyridine irritate our airways.
Toluene can damage brain cells and interfere with their development.
Concentration of chemicals in cigarettes
Most of the harmful substances in tobacco smoke are found at low levels in a single cigarette. But over months and years, many of them can build up to high levels in our bodies. And they are even more dangerous when mixed together.
This is why a smoker’s risk of cancer and other diseases increases the more cigarettes they smoke a day, and the more years they spend smoking.
Compared to non-smokers, the breath and blood of smokers can have:
twice as much cadmium,
four times as much radioactive polonium-210
ten times as much benzene
ten times as much arsenic
A major source of poisons
Due to strict regulations, we are mostly protected from exposure to the poisons in smoke in the environment.
So for most of us, a large part of our exposure to cancer-causing chemicals like benzene, formaldehyde and cadmium comes from breathing in tobacco smoke. And a few poisons, such as some nitrosamines, are unique to tobacco.
Some chemicals, such as arsenic and cadmium, are also found in some types of food but for various reasons, this is less of a problem:
Chemicals that we inhale are more problematic than those we eat, because our lungs are better than our guts at absorbing them. For example, our guts absorb about 6% of cadmium in our food, but our lungs absorb 60% of any inhaled cadmium.
Some chemicals, such as arsenic and chromium, are found in less harmful forms in food than in tobacco smoke.
Why do poisons build up?
Our cells have special cleaner proteins called ‘detoxification enzymes’ that mop up harmful chemicals and convert them into harmless ones. But the chemicals in smoke, such as cadmium, can either damage or overwhelm these cleaners. Because of this, it can take decades for your body to remove any cadmium that gets inside it.
Other chemicals such as formaldehyde, ammonia and hydrogen cyanide kill cilia, the small hairs that clean toxins from your airways.
This means that the poisons in smoke accumulate in our lungs, blood and other organs over time.
A cocktail effect
Individual chemicals in smoke can be even more dangerous when mixed together. They can interact with each other to increase our risk of cancer and other diseases even more.
For example, some chemicals in tobacco smoke can directly damage part of our DNA, including key genes that protects us against cancer. These chemicals include benzo(a)pyrene, polonium-210, benzene, acrolein and nitrosamines.
This is already bad news, but it’s made worse by other chemicals. Chromium makes poisons like benzo(a)pyrene stick more strongly to DNA increasing the chances of serious damage.
Arsenic, cadmium and nickel interfere with our bodies’ defence systems and prevent them from repairing damaged DNA. This makes it even more likely that damaged cells will eventually turn cancerous.
Source of the chemicals in cigarettes
The chemicals in cigarette smoke come from the tobacco plant itself, its surroundings, the manufacturing process, and burning the cigarette.
The tobacco plant itself
Nicotine is found naturally in the tobacco plant. It is a neurotoxin (a poison that kills nerve cells) and the plant uses it to stop animals from eating it.
Tobacco plants absorb various chemicals from the soil and fertilisers. These become stored in the leaves and are released when the leaves are burned. These chemicals include metals like cadmium, arsenic and chromium.
Tobacco plants have large leaves with sticky hairs called trichomes. These hairs can capture chemicals such as radioactive polonium-210 from the atmosphere, building up higher concentrations than other plants.
The leaves can also absorb and concentrate chemicals used in fertilisers and pesticides.
Some dangerous chemicals are formed when tobacco leaves are processed and cigarettes are manufactured.
For example, when tobacco is cured to remove moisture from the leaves, bacteria produce nitrites that react with chemicals in the leaves. This produces most of the nitrosamines in the final product. Many of these chemicals are unique to tobacco and not found in other plants.
Most of the dangerous chemicals in tobacco smoke are formed through the many chemical reactions that occur as the cigarette burns.
Burning organic material such as tobacco leaves produces polycyclic aromatic hydrocarbons. And sugars that are added to cigarettes produce formaldehyde when burned.
Hundreds of chemicals are added to cigarettes in order to make them taste nicer and easier to smoke. This include the irritating gas, ammonia, which can increase the addictive power of nicotine.
Additives in cigarettes
The tobacco industry adds hundreds of additives to cigarettes to make easier to smoke and more appealing.
Cigarettes contain a wide range of additives, which vary by brand and can include chocolate, vanilla, menthol, peppermint, sugar, liquorice, herbs and spices.
They may not be harmful themselves but they help to overcome the initial off-putting taste that new smokers often experience.
Masking the sight and smell of smoke
Internal documents from the tobacco industry, made available on the internet thanks to legal action in the USA, also reveal that it used additives to make second-hand smoke less irritating.
Some additives have been used to make the smoke less visible, while others mask the smell, including vanilla, cinnamon, coffee extract and nutmeg oil.
While these may make cigarettes more ‘socially acceptable’, they are not actually any safer.
Designing cigarettes for young people
In the 1970s, the tobacco industry began developing cigarettes specifically targeted at young people. These brands were designed to be less harsh and irritating.
Some companies used smoother tobacco at the filter end and stronger tobacco at the lit end. This gave a strong nicotine hit to start with but left the smoker with a ‘smoother’, more appealing taste.
The tobacco companies also added taste enhancers that make the cigarette seem smoother, including a combination of vanilla, chocolate and liquorice.
Smoking and cancer: What happens in your body?
Chemicals in cigarette smoke affect the entire body. This is why smoking causes so many diseases, including a dozen types of cancer, heart disease and various lung diseases.
The diagram on the right shows just some of the types of cancer that are caused by smoking.
As soon as you take a puff on a cigarette or breathe in someone else’s smoke, poisonous gases like formaldehyde will start to irritate your eyes, nose and throat.
Your lungs and airways
When you inhale the smoke, it damages the tissues of your airways and lungs. Chemicals like nitrogen oxide can constrict your airways, forcing your lungs to do more work and making breathing more difficult.
Hydrogen cyanide, carbon monoxide and ammonia weaken the natural cleaning mechanisms that clear your lungs and airways of toxins. This means that other dangerous chemicals, bacteria and viruses that you inhale stay inside your lungs.
Radioactive polonium-210 becomes deposited at the points where your airways split to connect to your lungs. This can subject local cells to much more radiation than they would otherwise experience.
From the lungs, cancer-causing chemicals and other poisons in tobacco smoke are absorbed into your bloodstream. These poisons are then carried to other parts of your body.
Your heart and blood vessels
Smoking and cancer: Filter and low-tar cigarettes
Filter and low-tar cigarettes make very little difference to the levels of chemicals in smoke. There is no such thing as a ‘safe cigarette’.
Some modern cigarettes have design elements that were originally thought to lower smokers’ exposure to dangerous chemicals. Unfortunately these changes don’t work. They include:
adding filters to cigarette tips
making brands with lower levels of nicotine and tar.
It is extremely unlikely that a ‘safe’ cigarette could ever be developed. The chemicals that are formed when tobacco is burned are naturally harmful.
The problem with filters
Filters may stop smokers from inhaling some of the solid particles in inhaled smoke. But they do not block out the many toxic gases in smoke, such as hydrogen cyanide, ammonia and carbon monoxide.
Filters also do nothing to reduce the chemical content of sidestream smoke. This is the smoke from the burning end of the cigarette, rather than the smoke that passes through the filters. Since second-hand smoke is mostly sidestream smoke, filters do not reduce non-smokers’ exposure to dangerous chemicals.
The problem with low-tar cigarettes
Cigarettes that are lower in tar also tend to be lower in nicotine. However, smokers who use filtered or low-tar cigarettes have ways of compensating unintentionally for this so that they can get the level of nicotine they need. These include:
taking longer drags
smoking more frequently
blocking filters with their fingers
This compensation is so strong that people who smoke ‘low tar’ cigarettes may end up inhaling as much or more smoke than those who smoke normal brands. Studies have shown that they could have similar or higher levels of cancer-causing chemicals in their bodies.
The tobacco industry
It is clear that the tobacco industry knew about how smokers compensated for low-tar cigarettes decades ago. We know this because of various documents released by the industry as a result of legal action taken against it.
The tobacco industry deliberately kept this information quiet and built a marketing strategy implying that low-tar cigarettes were a “safer” alternative for health-concerned smokers. Because this is not the case, the terms ‘light’ and ‘mild’ are no longer permitted in the EU for cigarettes.
Tar measurements taken with a smoking machine, were also known to be a poor reflection of the way people actually smoke and therefore of the actual amounts of tar inhaled. The tobacco industry’s own research showed that smokers inhale about twice as much smoke as a smoking machine.
Smoking and cancer: Why do people smoke?
People smoke for many different reasons. Smoking is very addictive because tobacco contains a powerful drug called nicotine. Smokers have also been influenced by the clever marketing tactics of tobacco companies for many years.
Nicotine as a drug
Cigarettes are deliberately designed to give you a fast nicotine hit. It takes just 10 seconds for the drug to reach your brain from inhaled cigarette smoke. Nicotine causes addiction in much the same way as heroin or cocaine. It is just as addictive as these ‘harder’ drugs.
Nicotine is a stimulant that increases your heart rate and affects many different parts of your brain and body. Smokers get a high because nicotine triggers the release of dopamine in the brain – a chemical linked to feelings of pleasure.
This also means that smokers start to make a mental link between the act of smoking and feeling good. Because of this, smokers can also become addicted to abstract things like the taste of cigarettes or the feeling of smoking, as well as the nicotine itself.
Addiction explains why giving up smoking can cause nicotine withdrawal symptoms. These symptoms include cravings, irritability, anxiety, difficulty concentrating, restlessness and disturbed sleep.
As your body adjusts to the lack of nicotine, these symptoms will start to disappear and most will go away within a month. Withdrawal symptoms can be difficult to cope but the benefits to your health are well worth it.
Nicotine as a poison
Nicotine is a neurotoxin (a poison that kills nerve cells) found in tobacco plants. It acts as a defence mechanism to stop them from being eaten by animals.
However, in cigarettes, the level of nicotine is too low to cause poisoning. And the nicotine in nicotine replacement therapy (NRT) is a safe way to come off the nicotine in cigarettes. Using NRT can double your chances of successfully quitting.
Tobacco advertising and promotion
Half of smokers die from smoking-related diseases. The tobacco industry needs new customers to replace the 114,000 people who are killed by smoking in the UK each year. Cigarette manufacturers make sure that:
they know exactly why people smoke
they cleverly market products to attract new customers.
In the past cigarette manufacturers have deliberately targeted children and young people. The industry spends a great deal of money on making cigarettes seem glamorous, appealing, fashionable and attractive. Most smokers started when they were young and image conscious. Young smokers often find it difficult to give up in later life.
Cigarette advertising is now banned in the UK. So the industry is developing new and subtle tactics to avoid prosecution.
Stress and relaxation
Many people claim that smoking helps them to cope with stress. But in fact, nicotine is a stimulant and won’t help you to relax. Smokers probably think a cigarette makes them feel better because when they aren’t smoking they suffer from nicotine withdrawal.
Other personal reasons for smoking
People have many other personal reasons for smoking. Smokers may:
use smoking as a support for when things go wrong
enjoy smoking with others as a shared activity
use smoking to start conversations and meet new people
smoke to make themselves look more confident and in control
think that cigarettes help them to keep their weight down
have a cigarette when they’re feeling bored or lonely
smoke when they need a break or a moment to themselves.
Smoking and cancer: Second-hand smoke
Breathing in other people’s smoke can cause cancer. Second-hand smoke can increase a non-smoker’s risk of getting lung cancer by a quarter, and may also increase the risk of cancers of the larynx (voice box) and pharynx (upper throat).
Second-hand smoke can cause other health problems too, including heart disease, stroke and breathing problems. Even 30 minutes of exposure to second-hand smoke can reduce blood flow in a non-smoker’s heart. Every year, second-hand smoke kills about 11,000 people in the UK from lung cancer, heart disease and strokes.
For more information about the evidence that links second-hand smoke to cancer, go to our How do we know section.
Second-hand smoke and children
Second-hand smoke is particularly dangerous for children because their bodies are still developing. Smoking when you are with your children can increase their risk of cot death, glue ear, respiratory illnesses such as asthma and chest infections, and possibly cancer later on in life.
A study by the Royal College of Physicians showed that about 17,000 children in the UK are admitted to hospital every year because of illnesses caused by second-hand smoke.
Over forty percent of children in the UK live in a household where at least one person smokes. If you are a smoker, try not to expose your child to your smoke.
The chemicals in second-hand smoke
There are two types of tobacco smoke:
mainstream smoke, which is directly inhaled through the mouth end of the cigarette, and
sidestream smoke, which comes from the burning tip of the cigarette.
Second-hand smoke consists mainly of sidestream smoke, which is about four times more toxic than mainstream smoke, although people inhale it in a more diluted form. This is because sidestream smoke contains much higher levels of many of the poisons and cancer-causing chemicals in cigarettes, including:
up to three times as much carbon monoxide
five times more cadmium
3-10 times more polycyclic aromatic hydrocarbons
10-40 times more nitrosamines
about 15 times more benzene
40-70 times more ammonia
Smoking in public places
In February 2006, MPs voted by a massive majority to make public spaces, including pubs and private members’ clubs, smokefree. This move will help to protect workers from the harmful effects of second-hand smoke and has been hailed as a large advance in public health.
Since the ban was brought into place, England has seen the largest ever fall in smoking rates. In the first year of the ban, about 400,000 people quit smoking and scientists have estimated that the new laws will prevent about 40,000 deaths from smoking-related diseases over the next decade.
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